Since its invention, radiation detectors such as Time Projection Chambers (TPCs) have been used successfully in many experiments to measure charged particles emitted in nuclear collisions, using devices such as the EOS TPC, the STAR detector at Relativistic Heavy Ion Collider (RHIC) and the ALICE detector at the Large Hadron Collider (LHC). For purposes of illustrating how a TPC operates, this disclosure will feature the SAMURAI Pion-Reconstruction Ion-Tracker Time Projection Chamber (SπRIT-TPC), designed for use with the SAMURAI spectrometer at the Radioactive Isotope Beam Factory (RIBF) at RIKEN, Japan. Those of skill in the art will recognize that the techniques disclosed here can be utilized with other gas radiation detectors, other similar devices and other TPC configurations.
The operation principle of a TPC and its wire planes are illustrated in FIGS. 1 and 2. FIG. 1 shows a TPC field cage, which is filled with counter gas. Electrodes on the walls of the field cage provide a uniform electric gradient potential within the cage. The TPC is normally placed inside a uniform magnetic field, which is parallel to the electric field. The magnetic field allows the determination of the momenta of charged particles and has the ancillary benefit of improving the resolutions of particle tracks by limiting the diffusion of electrons in directions perpendicular to the magnetic field.
When a violent heavy-ion reaction occurs, charged particles are produced in the target, which is located just outside of the upstream window of the field cage for the SπRIT TPC. These charged particles enter the field cage through the window and ionize the detector gas, liberating secondary electrons along the tracks of these particles. The secondary electrons drift along the anti-parallel electric and magnetic fields towards a set of three wire planes located at the top of the field cage. The wire planes are not visible in FIG. 1, but their functions are illustrated schematically in FIG. 2, in which the wires are drawn larger than scale to make them more visible. The 3 layers of wire planes are mounted just below a pad plane tiled with pads. This pad plane forms the upper boundary of the field cage volume, and the bottom boundary of the field cage is the cathode. By measuring the arrival time and the induced charge of the avalanched electrons produced around the anode wires, the TPC provides an accurate 3-D reconstruction of these tracks in the gas from which the particle momenta and the energy losses for each of the charged nuclear reaction products in the counter gas can be deduced.
In many TPC applications, there can be charged particles that enter the field cage that are not of scientific interest. In the SπRIT TPC experiments, these include beam particles that do not interact with the target or large projectile residues from very peripheral collisions. It is important to restrict gas multiplication of the ionization of such undesired particles. Amplification of undesired particles will accelerate the aging process of the anode wires by creating negatively charged polymers from the hydrocarbon components or impurities in the detector gas either during the primary ionization or during the avalanche. If the deposition of such polymers on the anode wires is not controlled, the effective anode wire diameters can increase with time due to the deposit, reducing the gas gain and deteriorating the performance of the TPC.